Credit card bills from holiday shopping aren’t the only charges that can surprise come January.

Chances are your utility bills will be higher, too, if you have holiday lighting, guests and spend more time watching TV or playing video games than usual.

San Diego Gas & Electric says that over the past three years, it has delivered about 100 megawatts more at its average peak in December than in January. The extra power used this month is equal to adding about 65,000 homes to the grid.

The reasons are not hard to fathom. We’re spending more time at home or entertaining.

Big TVs running for hours in front of the family soak up juice, as do new gizmos like computers, games and iPads.

And having overnight guests mean more cooking, showers, more hair drying, more lighting.

Oh, and yes, there’s the Christmas lights.

A 100-light string of mini lights will cost a total $1.75 to run eight hours a day for a month, at SDG&E’s average rate.

String a bunch together — and most people will, because what fun is just one strand of lights — and you’ll see an impact on your bill. (Add to this the fact that rates more than double when you use more power. More on that in a bit.)

Some people, of course, take it to the limit.

Since 1989, Bill Gilfillen has been building a huge holiday display outside his house in San Marcos. With 80,000 lights, moving figurines and more than 80 snow men, it draws thousands of visitors from Thanksgiving to Christmas every year and is featured on his website, christmasonknobhill.com.

It uses so much power that, after the circuit breakers blew one year, he had SDG&E come out to bolster his electrical supply.

“I use an awful lot of lights,” the 72-year-old Gilfillen said.

And that means an awfully big December electric bill, some $800 to $900 more than other months. Gilfillen admits it’s an indulgence, but compares it to the field lights at Qualcomm Stadium — something that uses electricity, but makes people happy.

He’s started putting in energy-efficient LED lights but hasn’t found as large a variety of figures as with incandescent bulbs, and he balks at the extra purchase expense.

“To get chasing LED lights, it’s really expensive,” he said. “If you balance out the two, the cost of running the lights, the electricity, more than likely, it would come out about the same.”

SDG&E promotes LED, or light-emitting diode, lighting, which uses a different, cooler technology that also reduces the risk of fires.

“When you use an LED strand, you save 90 percent (in energy costs),” said SDG&E spokeswoman April Bolduc.

Early each holiday season, the power company exchanges incandescent strands for LED ones at events around the county. If you didn’t take advantage this year, there’s always next year.

While they’re cheaper to run, LED lights cost more to buy than incandescent ones. At Target.com, for instance, you could get a strand of 60 LED Christmas lights for $11.99 this week, while a strand of 300 incandescents was selling for $19.99, or one-third the cost.

Recently, San Diego’s EcoDog took a strand of white LED rope light and compared it to a strand of small white incandescent lights and found the LEDs use about one-fifth as much power.

The company, which makes home energy monitoring equipment and software, then took the results and extrapolated what it would mean for people’s bills, which is tricky because, as noted, residential rates are much higher if you use a lot of power.

Those who are able to stay below “baseline” would pay 74 cents to power a strand of 100 incandescents for a month versus 19 cents for LEDs. Those who use the most power would pay $1.53 for the incandescents versus 38 cents for the LEDs.

“We figure most folks will be better off using their old lights until they die and then buying LEDs as replacements in strictly financial terms,” said EcoDog spokeswoman Susan Connell. “For those wanting to accelerate their greenness, the LEDs clearly use a lot less energy.”

For Randy Schimka of San Diego’s Tierrasanta neighborhood, the switch to LEDs wasn’t just about the money. He also has a mondo display, more than 80,000 lights, which he started building six years ago. Now, it’s more than 90 percent LED.

“The colors are so brilliant,” he said. “You can’t get certain colors out of the incandescent lights, especially blue,” he said.

His light display — a labor of love he dedicates to his autistic sons — is computer-controlled and is timed to match music he broadcasts out of a radio transmitter in his garage at 5306 Belardo Drive. (Learn more at belardolights.com.)

The blinking means that at any given time, about half of the lights are on. That translates into lower energy use.

The switch to LEDs has also meant that he can add more lights without having to upgrade his electrical system.

“There’s motivations for switching to LEDs on the display, but the power bill is not really the motivation,” said Schimka, who works for SDG&E.

LEDs have a drawback that makes them less than green. They contain lead and other heavy metals that can damage the environment when you throw them away, according to a University of California Davis study.

Researchers there say manufacturers should work to limit toxic content, and users should be careful about disposal.